1. Field of the Invention
This invention relates to rubber compositions particularly for use in tread portions of automobile tires.
2. Prior Art
To cope with the advent of sports cars of high performance, a great concern has been directed to the driving stability of tires at high speeds. Tread rubbers have to this end been developed which are so formulated as to exhibit increased tan .delta. at 60.degree. C. This particular quality is closely associated with tire gripping, i.e. frictional resistance between the tire tread and the road surface while in tire running. Improved tire gripping is generally dominated by increased tan .delta. at 60.degree. C.
Taking the criticality of tan .delta. in view, many attempts have been made with elastomeric materials such as styrene-rich or vinyl-rich styrene-butadiene rubbers (SBR) of high glass transition temperatures (Tg). Alternatively, large amounts of carbon black have been incorporated in base rubbers. Because of their high Tg temperatures, however, such SBR rubbers will usually depend in nature upon temperature and thus tend to invite reduced hardness at elevated temperature. This in turn leads not only to insufficient rigidity over a temperature range of 20-100.degree. C. within which to run the tire but also to inadequate breaking strength and poor abrasion resistance, meaning that those known SBR rubbers will not warrant commercial application. Too much carbon black fails to uniformly disperse in a given rubber mix and if not, will in most cases render the mix susceptible to great hysteresis loss and hence objectionable heat buildup with the results that various necessary dynamic properties get impaired.